In all honesty, Id steer clear of disposable systems entirely as theyre extremely expensive to run on any decent sized tank. A refillable system, which is what im guesing you meant, isnt that expensive, you can always get a solenoid later and just turn the co2 off/on by hand, or do as some do and leave it on 24/7 at lower levels. the regulators for disposable/refillable systems are very similar in price anyway.. then all you have to do is find a cheap fire extinguisher off ebay, should be about £30.
If you dont already have the D-D system, id be wary, I had 2 before i knew what i was doing and both were faulty, dumping the entire cylinder over night. and were very expensive for the build quality.